Semiconductor Thin Films Research Articles

Influence of Anisotropy of Isoenergetic Surface on Electrical Conductivity and the Hall Constant for a Thin Semiconductor Film

Influence of Anisotropy of Isoenergetic Surface on Electrical Conductivity and the Hall Constant for a Thin Semiconductor Film

Electrical properties of zinc nitride and zinc tin nitride semiconductor thin films toward photovoltaic applications

Abstract Zn3N2 (ZN) and ZnSnN2 (ZTN) are a promising class of nitride semiconductors for photovoltaic and light-emitting-diode applications due to their particular electrical and optical properties, elemental abundance and non-toxicity. So far, most of the experimental results show the degenerate carrier concentration. However, we find that low-temperature growth of these films in a chamber with ultra-high background vacuum can attain a non-degenerate electrical conductivity. This work provides the recent progress of the electrical properties of ZN and ZTN semiconductor thin films. The origins for the high carrier concentrations in ZN and ZTN have been discussed, demonstrating that non-intentional oxygen and hydrogen-related defects play significant roles in such high carrier concentrations. The strategies to suppress the carrier concentrations have also been addressed, such as ultra-high vacuum conditions and low temperature growth.

Mechanically Contacted Distributed-Feedback Optical Microcavity.

We report a construction of distributed-feedback (DFB) optical microcavities, which is realized through mechanical contact between a high-quality planar thin film of a polymeric semiconductor and a large-area homogeneous nanograting. Using poly[(9,9-dioctylfluorenyl-2,7-diyl)-alt-(benzo[2,1,3] thiadiazol-4,8-diyl)] (F8BT) as the active medium for the planar layer, we achieve strong amplified spontaneous emission from such a microcavity with a low threshold. This not only simplifies largely the fabrication techniques for DFB microcavities, but also avoids the unexpected chemical interactions during solution processing between the organic semiconductors and the nanograting materials. Furthermore, high-quality polymer thin films with high surface smoothness and high thickness homogeneity are employed without any modulations for constructing the microcavities. This also suggests new designs of microcavity light-emitting diodes, or even for realizing electrically pumped polymer lasers, simply by metallizing the dielectric nanogratings as the electrodes.

Flexible Perovskite and Organic Semiconductor Heterojunction Devices for Tunable Band-Selective Photodetection

Filter-free band-selective photodetectors with tunable band edges possess extensive applications in smart sensors, artificial intelligence, the internet of everything, and so forth. However, such photodetectors operated with the charge collection narrowing effect require a thick active layer over millimeters, leading to rather a large device size, high material cost, and limited mechanical flexibility. In this work, we report a flexible thin-film perovskite and organic semiconductor (OSC) heterojunction device, which can achieve adjustable band-selective photodetection by utilizing the easy tunability of the perovskite band gap. First, heterojunction photodiodes with only two active layers of perovskites and OSCs are fabricated, in which the perovskite contributes to light absorption and rectification characteristics simultaneously. The fundamental device operation mechanism is investigated by combining the band alignments of the heterojunctions and their potential distributions observed with Kelvin probe force microscopy. It uncovers that the presence of built-in electric fields in the heterojunctions caused by moderate band alignments and appropriate difference in hole and electron concentrations between the perovskites and the OSCs is significant for the properties of the photodiodes. The resultant photodiodes exhibit a high rectification ratio of up to 103, decent photodetection performance, and mechanical flexibility. Furthermore, photodetectors based on perovskite/OSC/perovskite stack heterojunctions can realize the functions of filterless band-selective photodetection in a single device, and the band edges in the photodetection can be modulated by tuning the perovskite band gap. Therefore, this work provides flexible thin-film photodiodes with the potential of miniaturization and low-cost fabrication and demonstrates tunable band-selective photodetection for advanced applications.

Driftfusion: an open source code for simulating ordered semiconductor devices with mixed ionic-electronic conducting materials in one dimension

The recent emergence of lead-halide perovskites as active layer materials for thin film semiconductor devices including solar cells, light emitting diodes, and memristors has motivated the development of several new drift-diffusion models that include the effects of both electronic and mobile ionic charge carriers. In this work we introduce Driftfusion, a versatile simulation tool built for modelling one-dimensional ordered semiconductor devices with mixed ionic-electronic conducting layers. Driftfusion enables users to model devices with multiple, distinct, material layers using up to four charge carrier species: electrons and holes plus up to two ionic species. The time-dependent carrier continuity equations are coupled to Poisson’s equation enabling transient optoelectronic device measurement protocols to be simulated. In addition to material and device-wide properties, users have direct access to adapt the physical models for carrier transport, generation and recombination. Furthermore, a discrete interlayer interface approach circumvents the requirement for boundary conditions at material interfaces and enables interface-specific properties to be introduced.

Improved Optoelectronic Characteristics of Ga-In co-Doped ZnO UV Photodetectors by Asymmetric Metal Contact Structure

Transparent Ga and In co-doped ZnO (ZnO:Ga-In) semiconductor thin films were deposited on Corning glass substrates by the sol-gel spin-coating process. The ZnO:Ga-In thin films were used as the sensing layer of metal–semiconductor–metal (MSM)-type ultraviolet (UV) photodetectors (PDs). In this study, the optoelectronic characteristics of ZnO:Ga-In MSM PDs with symmetrical interdigital electrodes (Al–Al) and asymmetrical interdigital electrodes (Al–Au) were compared. The as-prepared ZnO:Ga-In thin films were polycrystalline, and they had a single-phase hexagonal wurtzite structure and high transparency (~88.4%) in the visible region. The MSM-PDs with asymmetric electrodes had significantly reduced dark current (9.6 × 10−5 A at 5 V) according to the current-voltage (I-V) characteristics and higher photoresponse properties than those of the MSM-PDs with symmetric electrodes, according to the current-time (I-t) characteristics. In addition, the Al–Au devices were self-powered without an applied bias voltage. The photocurrent was 6.0 × 10−5 A; the sensitivity and responsivity were 0.25 and 0.03 mA/W, respectively, under UV illumination.

Magnetron-sputtered gallium–magnesium–zinc oxide transparent semiconductor thin films: Structural, optical and electrical investigation

Thin transparent semiconductor films of gallium–magnesium–zinc oxide (GaMgZnO) have been prepared with magnetron-sputtering technique on quartz glass substrates. We have investigated the effects of sputtering pressure on the crystallization, micro-structure, electrical and optical features of the deposited films. The results indicated that all the deposited films were polycrystalline in nature having a hexagonal wurtzite-type crystal structure with a preferred grain orientation in the (002) direction. The sputtering pressure has a significant impact on the structure and electro-optical characteristics of the deposited films. The sample prepared under sputtering pressure of 3.6 Pa possesses the best crystalline quality and electro-optical performance, with the minimum dislocation density (3.69×1014linesm−2), tensile stress (8.26×107 Pa), resistivity (1.53×10−3Ωcm), the maximum mean visible transmittance (87.71%) and the highest figure of merit (4.73×103Ω−1cm−1). Additionally, the external extrapolation method is employed to evaluate the optical band gap. The gap value is between 3.41 and 3.44 eV, which is larger than the optical band gap of pure ZnO.

Performance improvement in amorphous zinc tin oxide ultraviolet photodetectors using an asymmetric pair of interdigitated electrodes

Solar-blind ultraviolet (UV) photodetectors with an amorphous zinc−tin oxide (a-ZTO) sensing layer have been fabricated on alkali-free glass substrates by a sol-gel method and spin coating technique. The as-prepared a-ZTO semiconductor thin films are highly transparent (>88.5%) in the visible region and exhibit moderate electrical properties at room temperature. The effects of changes in the asymmetry ratio of the line-width of a pair of Al interdigitated electrodes on the photoelectrical characteristics of solution-processed a-ZTO metal−semiconductor−metal (MSM) UV photodetectors was investigated. These photodetector devices had good photosensing properties, and their current-voltage (I–V) characteristic exhibited obvious rectification behavior under UV light illumination when operated at bias voltages of -3 V–3 V. Experimental results showed that the responsivity and specific detectivity of the a-ZTO MSM photodetectors increased with increases in the asymmetry ratio of a pair of Al interdigital electrodes. The a-ZTO MSM UV photodetector operated at 3 V bias voltage had the best responsivity of 7.42 mA/W and the highest specific detectivity of 6.35 × 107 J when the asymmetry ratio of electrodes was 10:1.

Hole-injection role of solution-processed thermally treated VOx thin films in Si nanowire-based solar cells

We have studied the hole-injection workability of dopant-free vanadium oxide (VOx) semiconductor thin films. Solution-processable VOx films were annealed in different atmospheres at 450 ˚C. X-ray diffraction showed that different crystal structures of V2O5, VO2 (M) and VO2 (B) were obtained after annealing in oxygen, nitrogen and under vacuum respectively. Gap states that originated from oxygen deficiencies located inside the band gap of the annealed VOx were demonstrated using X-ray photoelectron spectroscopy. These VOx thin films act effectively as transparent hole-injection layers owing to their high work function, wide band gap, and presence of oxygen deficiencies. The different ambient annealed VOx films were employed in Si nanowire (SiNW)-based solar cells. The vacuum-annealed oxide film VO2–x (B) showed the best hole-injection ability. SiNWs/VOx solar cells are noticeably more stable than SiNWs/PEDOT:PSS hybrid solar cells, which decay swiftly under ambient conditions unless encapsulation is provided. The utilization of VOx films to act as hole-injection layers opens a pathway toward the development of cheap and durable electronics and optoelectronics devices.

Estimating thin-film thermal conductivity by optical pump thermoreflectance imaging and finite element analysis

We introduce a noncontact experiment method to estimate thermal conductivity of nanoscale thin films by fitting high spatial resolution thermoreflectance images of surface spot heating to a finite element simulated temperature distribution. The thin-film top surface is heated by a 1μm diameter focused, 825 nm wavelength laser spot. The surface temperature distribution in the excited sample is imaged by thermoreflectance microscopy with submicrometer spatial resolution and up to 10 mK temperature resolution. Thin-film thermal conductivity is extracted by fitting a measured surface temperature distribution to a 3D finite element temperature model. The method is demonstrated by estimating thermal conductivity for an isotropic thin-film metal (nickel, 60–260 nm) on a glass substrate. The fitted Ni thermal conductivity was 50 ± 5 W/m K, which is in good agreement with the literature. Also, we present a detailed finite element analysis for an anisotropic thin-film semiconductor sample to show how the method could be extended to estimate thermal conductivity of anisotropic thin films. Advantages of the new method are easy sample preparation (no top surface transducer film or integrated heater required), rapid in situ measurement, and application to a broad range of thin-film materials.

Directional and Polarized Lasing Action on Pb‐free FASnI3 Integrated in Flexible Optical Waveguides

AbstractIn this work, high‐quality FASnI3 (FA, formamidinium) lead‐free perovskite thin films are successfully incorporated in a flexible polyethylene terephthalate (PET) substrate to demonstrate amplified spontaneous emission (ASE) and lasing. The waveguide (WG) consists of polymethylmethacrylate(PMMA)/FASnI3 bilayer deposited on a PET substrate and is properly designed to allow single‐mode propagation at the photoluminescence wavelength. This geometry optimizes the excitation of the emitting FASnI3, enhances the light−matter interaction in the semiconductor thin film, provides a preferable direction for the emitted light and allows its direct outcoupling for on‐chip or fiber‐optic applications. As far as the authors know, ASE and random lasing are obtained for the first time in a flexible‐based WG integrating a highly efficient lead‐free perovskite. The high quality of the deposited films and the optimized design of the structure result in an extremely low ASE/lasing threshold in the range of 1 µJ cm−2, which is only ten times higher than that measured in the same PMMA/FASnI3 structure deposited on a rigid substrate (Si/SiO2). More interestingly, these WGs exhibit a strong polarization anisotropy for the outcoupled ASE/lasing light with a preferable transverse electric polarization. This work is the base for the future development of ecofriendly, flexible, and efficient photonic devices.

A Doped Hole Transport Layer Qualified for 100 °C‐Tolerant Perovskite Solar Cells

AbstractDue to the extremely low intrinsic carrier density and disordered nature of organic semiconductor thin films, unintentional or intentional chemical doping is inevitable for sufficiently high carrier density and charge mobility unless charges are either injected from appropriate electrodes or by photoexcitation. However, doping often results in a reduction in thermal tolerance. The charge transport layers for thermostable perovskite solar cells (PSCs) need to possess a heat‐resistant morphology and to suppress the diffusion of volatile species. Here the authors report a molecular semiconductor with a molecular mass of >2000 Daltons but free of any flexible side chains. Its air‐doped composite with thermally stable acid salt exhibits an electrical conductivity of over 50 μS cm−1 and a glass transition temperature of 142 °C. Molecular dynamics simulations reveal that molecules and ions in this composite do not experience significant translational motion even at 150 °C. These unusual features allow for the fabrication of 21.5%‐efficiency PSCs that can withstand 200 h at 100 °C.

Drain Current Drop in Oxide Semiconductor Thin-Film Transistors: The Mechanisms and a Solution

The reliability of amorphous oxide semiconductor thin-film transistors (AOS-TFTs) is vital for high-definition displays and functional electronic devices. However, drain current drop (DCD) degradation has been commonly observed in the output characteristics of thin-film transistors (TFTs) with channel lengths of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${L} &lt; 10 \mu \text{m}$ </tex-math></inline-formula> . Here, we show that DCD is a reversible process that is closely related to reduced metal-oxygen bonds. Based on device theory and simulations, we propose that the DCD effect is mainly caused by hot carriers, whose velocities are inversely proportional to the drain depletion width. Therefore, DCD failure could be alleviated simply by using the drain-offset structure. Experiments show that drain-offset TFTs improve DCD critical voltages (by ~130%) without sacrificing the ON-/ OFF-ratio. These studies provide theoretical and experimental approaches to effectively suppress DCD.

Room temperature ferromagnetism in Fe3O4 nanoparticle-embedded polymer semiconductors

We succeeded in efficiently fabricating polymer magnetic semiconductor thin films composed of poly(3-hexylthiophene-2,5-diyl) (P3HT) and Fe3O4 magnetic nanoparticles at ambient temperature. A strong magnetic signal was observed with a magnetic force microscope after the application of a magnetic field to the P3HT film with embedded Fe3O4 magnetic nanoparticles. The polymer magnetic semiconductor had an obvious ferrimagnetic hysteresis curve, which was measured using a superconducting quantum interference device at room temperature, and its coercivity was 300 Oe, which indicated strong spin coupling between P3HT molecules and Fe3O4 magnetic nanoparticles.

Accurate Polarization-Resolved Absorption Spectra of Organic Semiconductor Thin Films Using First-Principles Quantum-Chemical Methods: Pentacene as a Case Study.

Theoretical studies using clusters as model systems have been extremely successful in explaining various photophysical phenomena in organic semiconductor (OSC) thin films. But they have not been able to satisfactorily simulate total and polarization-resolved absorption spectra of OSCs so far. In this work, we demonstrate that accurate spectra are predicted by time-dependent density functional theory (TD-DFT) when the employed cluster reflects the symmetry of the crystal structure and all monomers feel the same environment. Additionally, long-range corrected optimal tuned functionals are mandatory. For pentacene thin films, the computed electronic spectra for thin films then reach an impressive accuracy compared with experimental data with a deviation of less than 0.1 eV. This allows for accurate peak assignments and mechanistic studies, which paves the way for a comprehensive understanding of OSCs using an affordable and easy-to-use cluster approach.

Robustness of organic physically unclonable function with buskeeper circuit for flexible security devices

Flexible devices have been studied to realize IoT or novel wearable devices. The data that flexible devices deal with can include personal information when application areas further expand. A security system for flexible devices becomes more important in this case. In this study, we investigate the thermal stability of an organic flexible security system. The security system utilizes fabrication variation of a chip, and it is called a physically unclonable function (PUF). As a result, the bit error rate of the organic PUF is 1.8% and the index of ID uniqueness (i.e., randomness) has an almost theoretical value of 0.48. The generated ID remains even after annealing at 100 °C for 97 h by using CYTOP encapsulation. X-ray diffraction measurement implies that degradation of PUF characteristics partially derives from structure changes of the organic n-type semiconductor thin film after annealing.

Near-field mapping of high permittivity dielectric microwave resonator modes via optically induced conductance.

In this paper, we demonstrate a straightforward, low-cost, and high resolution optical-based method to measure the three-dimensional relative electric field magnitude in microwave circuits without the need to monitor reflected laser beams or the requirement of photoconductive substrates for the device under test. The technique utilizes optically induced conductance, where a focused laser beam excites electron-hole-pairs (EHPs) in a semiconductor thin film placed in the near-field of a microwave circuit. The generated EHPs create localized loss in the resonator and modulate the transmitted microwave signal, proportional to the local microwave electric field. As a proof of principle, several different modes of a high permittivity (ɛ ∼ 80) cylindrical dielectric resonator are mapped.

Design, Growth, and Characterization of Crystalline Copper Oxide p-Type Transparent Semiconductive Thin Films with Figures of Merit Suitable for Their Incorporation into Translucent Devices

Design, Growth, and Characterization of Crystalline Copper Oxide p-Type Transparent Semiconductive Thin Films with Figures of Merit Suitable for Their Incorporation into Translucent Devices

Experimental and theoretical study of hole scattering in RF sputtered p-type Cu2O thin films

One of the key materials of interest for p-type oxide semiconductor thin film electronics is cuprous oxide (Cu2O), due to its relatively high hole mobility. In this work, we use experiments, analytical models, and density functional theory calculations to study the scattering mechanisms that determine Hall mobility in two Cu2O samples. First, we examine a polycrystalline Cu2O thin film deposited by RF magnetron sputtering, and second, a single-crystalline Cu2O bulk substrate. Temperature-dependent Hall measurements indicate that neutral impurity and grain boundary scattering are dominant for the polycrystalline Cu2O thin film, while phonon scattering is dominant for single-crystalline Cu2O. Our first-principles calculations show that the room-temperature intrinsic hole mobility of Cu2O is 106 cm2 V−1 s−1, indicating the great promise of the material for p-type electronic devices. This intrinsic mobility is limited by phonon scattering, with the most dominant scattering modes having phonon energies of 88.4 and 17.1 meV. These results indicate that the key pathways to increase the hole mobility in Cu2O thin films are by reducing the impurity concentration and by increasing grain size. Our work thus sets the stage for the future development of high performance Cu2O-based p-type thin film transistors.

Temperature-induced potential barriers in piezoelectric semiconductor films through pyroelectric and thermoelastic couplings and their effects on currents

We study temperature effects in piezoelectric semiconductor thin films through pyroelectric and thermoelastic as well as piezoelectric couplings. Two-dimensional equations based on the macroscopic theory of piezoelectric semiconductors for extensional deformations of the films are used. A p-type ZnO film with an in-plane c-axis is analyzed. It is shown through a linear and theoretical analysis that a local temperature change produces a pair of a potential barrier and a well in the film. A nonlinear numerical analysis is then performed when a voltage is also applied, showing that the potential barrier blocks the in-plane motion of the holes and thus affects the current density distribution. The effects of various physical and geometric parameters are examined. The results obtained provide the theoretical foundation for thermally manipulating currents in piezoelectric semiconductor films for piezotronic applications.